The present invention relates generally to a system for the activation of precious metal containing monomers or comonomer precursors utilizing plasma polymerization techniques. The monomer or comonomer precursors are positioned within the glow zone for conversion to the dissociated form wherein the precursors are enriched with radicals, electrons and ions (i.e., plasma conversion), and deposited as a platinized coating. The platinized coating is further characterized by its being deposited as a plasma polymer or copolymer onto and/or into an appropriate substrate also disposed within the glow zone. Although a variety of substrates are useful, the plasma polymerization operation also preferably includes utilization of a porous electrically insulative substrate such as one consisting of glass. In accordance with the present invention, the entire process including each of the individual operations is undertaken at low temperatures, and specifically at or about room temperature. The features of the present invention permit these operations to be undertaken without need to heat either the source percursor or the substrates employed in the process.
Presently known methods utilized for making films of noble metals such as platinum, gold platinum, ruthenium and alloys such as platinum and palladium, or other precious metals or alloys including for example alloys of platinum/copper and platinum/tin do not appropriately and completely cover the entire surface of a porous structure including porous membranes, beads, aerogels, fibers, porous silicon, or others. The present invention is distinguishable inasmuch as it utilizes chemical vapor infiltration followed by low temperature, low energy plasma induced metallization for the application of continuous layers or films of precious metals and/or alloys. Conventional vapor state plasma discharge operations, for example, has been found to be ineffective for pore penetration of the films, and in particular is ineffective for use with a variety of platinum compounds. Additionally, the size of the particulate obtained from conventional vapor state plasma fails to be generally in nanometer dimensions.
In accordance with the present invention, an electrical discharge from an RF generator is applied to the external electrodes of a capacitively coupled tubular plasma reactor. The articles are initially pre-exposed to the vapors. Thereafter, the selected monomer or comonomer precursors are introduced along with a carrier gas into the reactor and energized into a plasma. The area of highest plasma energy density within the reactor is controllable, and typically controllably disposed in the area between the electrodes and the plasma glow zone.
While positioned within the plasma glow zone, the substrate is preferably rotated in order to allow the substrate to receive a uniform deposit of plasma polymer. As an alternate procedure, the electrodes may be moved to accommodate unusual configurations, such as elongated substrates or the like. In accordance with the present invention, it is not necessary that components be held at elevated temperatures, and hence the operations may proceed at low temperatures, such as at or about room temperature.
A preferred material for use in connection with the present invention is platinum (II) hexafluoroacetylacetonate. The hexafluoroacetylacetonate compound is commercially available and is further identified through CAS #65353-51-7 FW=609.22.
One system for the continuous production of the platinized glass substrate employs an RF plasma reactor of tubular configuration. This reactor employs a pair of capacitively coupled external electrodes positioned at either end of the reactor, and is externally coupled to an RF generator. The highest energy density is maintained in the area between the electrodes, that is, the plasma glow zone, by controlling both the current from the RF generator, the gas supporting the plasma, and optionally, the monomer or comonomer flow rate. In this situation, when the flow rate becomes too rapid, the glow zone will xe2x80x9cspill overxe2x80x9d to the region outside of the electrodes. On the other hand, if the flow rate is too slow, the plasma will fail to ignite or will fail to fill the entire inter-electrode region. The chambers employed are in vacuum-sealed relationship, and each is provided with an outlet to a vacuum pump. The reactor chamber may be formed of any material with sufficient resistance to withstand the plasma polymerization reaction condition. Preferably materials found suitable for this application are quartz, Pyrex(trademark) and Vycor(trademark). In addition, certain plastics have been found satisfactory as well as various ceramics.
Proper spacing between the electrodes in the plasma reactor depend upon the size of the reactor tube. One system which has been found useful employs electrodes approximately 10-15 centimeters apart, with the tube having a diameter of 25 mm. When larger diameter tubes are employed, the energy density associated with the plasma glow zone should be maintained as closely as possible to that in the smaller tubular reactor. Along with energy density, it is extremely important to control the density of the monomer as well as the density of the plasma carrier gas. In this instance, the monomer is platinum (II) hexafluoroacetylacetonate. Although monomer density will generally remain the same with changes in tube size, some variation in the optimum of both energy density and monomer density will result with changes in system size and design. In the system described herein, the current from the RF generator is preferably maintained at an appropriate level for the application. Depending upon the size and the configuration, those of skill in the art are able to ascertain the appropriate power level. Other conditions are as follows:
Frequency=13.56 Megahertz.
This frequency is appropriate for this application, and is authorized for use by the Federal Communications Commission. Other frequencies can be employed, particularly lower frequencies in the kilohertz range.
Methods for plasma generation between electrodes using an electric field are well known in the art. A DC field, or an AC field from 50 Hz to about 10 GHz are typical. Power values ranging from between about 1 watt to 5,000 watts are suitable.
A preferred electrical field generating means for plasma processing is the use of a high frequency power supply to initiate and sustain the plasma. The preferred operating frequency is 13.56 MHz. Other frequencies, such as 75 KHz are sometimes employed. The particular frequency and power values chosen depend on the deposition requirements of the coating materials and substrates.
Also well known in the art are potentially beneficial modifying means for increasing the ionization potential and/or providing improved spatial control of the plasma through the use of separate magnetic fields, i.e., electron cyclotron resonance (ECR) microwave plasma technique.
A useful guide in determining changes in reaction parameters with changes in tubular geometry is the composite discharge parameter W/FM, where xe2x80x9cWIxe2x80x9d is the plasma wattage, xe2x80x9cFxe2x80x9d is the flow rate of the monomer or monomers, and xe2x80x9cMxe2x80x9d, is the molecular weight of the monomer or monomers. As a tubular geometry and system size vary, W/FM may vary for a given plasma polymer or copolymer deposition rate, but optimum W/FM will vary between one-half to twice that of the original W/FM for a given monomer system. Therefore, for a given monomer system, changes in the composition plasma parameter with changes in tubular geometry may be expressed:
(xc2xdWa/FaMa less than WbFbMb less than (2)Wa/FaMa
where Wb/FbMb is the complete plasma parameter for a first tubular RF tubular reactor as described herein, and where Wa/FaMa is the composite plasma parameter for a differently sized tubular reactor as described herein.
While this reactor design has been found useful, other designs will, of course, be capable of application to the process as well.
The arrangement can readily be adapted to continuous production or treatment of flat films, discs, rods, tubes, filaments, beads, or other articles by providing a suitable reactor chamber with supply and take-up spools or fixtures to accommodate the workpieces.
In accordance with the preferred embodiment of the present invention, platinized articles such as platinized membranes are formed utilizing porous articles such as glass and the like as the substrate. In addition to platinum compounds, other noble metal coatings may be prepared in a similar or related fashion. The operation includes the steps as set forth below.
After selection of the substrate, the surface of the substrate is cleaned. In this example, hollow glass tubes are employed, with these tubes being characterized as porous Vycor(trademark) glass. Porous Vycor(trademark) glass (thirsty glass) is initially treated to remove impurities which have been absorbed from contaminating vapors in the atmosphere.
The following cleaning steps are initially undertaken:
1(a). Dip them in concentrated nitric acid and heat the nitric acid bath to 100xc2x0-110xc2x0 C. for approximately 4 to 6 hours. Cover the bath with a glass cover having provision for escape of vapors.
1(b). Remove the Vycor(trademark) porous glass, when clear, from the bath with the help of a glass rod and wash several times in distilled water.
1(c). Exchange the distilled water with absolute alcohol by dipping the articles in absolute alcohol solution for approximately 30 minutes.
1(d). Remove the glass articles from alcohol solution and place into a vacuum chamber to evacuate overnight or until the pressure drops to less than 20 millitorr.
Thereafter, the following steps are undertaken:
2. Expose freshly dried, clean Vycor(trademark) to the vapors of platinum (II) hexafluoroacetylacetonate (hereinafter sometimes referred to as PtHFAA) CAS#65353-51-7 FW=609.22 preferably without exposure to air. This can be done by keeping 300 to 500 mg monomer in a glass petri dish in a feed chamber next to the cleaned Vycor(trademark) glass articles which in turn are mounted onto fixtures (e.g. tubes onto rods). Make sure to make provision for uniform passage of monomeric vapors over the glass articles. Alternatively, the monomeric vapors may be bled into the chamber via control valves connecting a source vessel of PtHFAA.
3. In the case of the petri dish above, evacuate the vacuum system preferably to 20 millitorrs or less. Initially, the pressures will be high as vapors emanate from the PtHFAA powder.
4. In the case of Vycor(trademark)7930 hollow tubes, the glass membranes start turning yellow within one to two minutes. In some instances, a greater exposure is helpful, such as up to about ten minutes. Continue exposure for 4 to 5 hours or until all the tubes have become deep yellow in color. The final pressure will be 20-25 millitorrs approximately. Less exposure is appropriate for thinner platinized coatings, but care must be taken not to exposure the Vycor(trademark) (or other materials) to atmospheric contaminants.
5. Release vacuum by Argon flushing.
6. In the case of tubes, mount the tubes onto mandrels or mounting assemblies which position them appropriately within the reactor. The reactor tube should be clean because deposited metal from prior exposures may interfere with the platinization process. Thus, disposable reactor tubes or sleeves covering the inside of a reactor vessel are useful.
7. The remaining Vycor(trademark) tubes (minus monomer) can be left in the feed chamber. The remaining monomer should be stored in a desiccator or, in the case of a valved system, stored under conditions preventing evacuation and/or contamination. Other inert storage conditions will be acceptable and are well known in the art. Room temperature storage is possible, provided the Vycor(trademark) articles are fully pre-exposed to PtHFAA. Less than full exposure (i.e., non-saturated conditions) will require re-exposure before platinization.
8. Evacuate the reactor system to 20 millitorrs. In a small reactor system with limited pumping capacity this may take 1 to 3 hours. In larger systems, it may take 10 to 30 minutes.
9. Position the RF capacitive electrodes. In this example, positioning of copper electrodes was at 9 cm from feed inlet reactor side, and 6 cm from feed outlet, with an inter-electrode gap of 16 cm.
10. Initiate flow of Argon into the reactor. In the example, a set point of 10 (20 SCCM CH4 mass flow meter) corresponding to an absolute value of approximately 3.83 SCCM was chosen. The pressure was allowed to stabilize at 120-130 millitorrs in the feed chamber measurement. As is known in the art, the mass flow will need to be adjusted in larger reactors such that a comparable pressure is achieved in any given volume. The plasma energy density will also need to be considered so that an increased power input level suitable to match the larger mass of gaseous species is obtained.
11. Plasma flow is initiated, sometimes with the assistance of a corona discharge gun. The power input was set at 10 watts with zero reflected power.
12. The Argon plasma treatment is continued for 10 to 15 minutes. Depending on the power input level and the mass flow rate of Argon chosen, the pre-plasma glow pressure and the glow discharge pressure can be varied over a wide range 60-200 millitorrs. However, the lower pressures appear to result in less complete platinization and the higher energy and/or pressure levels result in excess heat with no noticeable improvements in final properties of the articles produced.
13. Ideally, the articles should be rotated or moved through the plasma in such a way as to help ensure uniform platinization over the exposed surfaces. After a bright shiny metallic surface is clearly evidenced, continue the glow treatment for another 5 minutes to help burn off residual unwanted carbon. When the interior of a tube is to be platinized as opposed to the outside, or where a layer of platinum is to be located at a specific depth in the wall of a tube or disc, etc., the techniques known in the art for xe2x80x9csingle sidexe2x80x9d or xe2x80x9ccounter flowxe2x80x9d low pressure chemical vapor deposition (LPCVD) of silicon dioxide films from various silanes can be modified to obtain a controlled platinization layer or zone within the interior wall of the substrate tubing, for deposition at a specific location on or along the wall. It is to be noted that the techniques of the present invention permit procedures to move forward under room temperature conditions, it being noted that commonly available techniques require the operations to be undertaken with components that are held at elevated temperatures.
14. Upon achieving the degree of platinization desired, the power is shut off and the system is vented against Argon.
15(a). Alternatively, the articles may be sequentially coated first via this platinization process, and then with a subsequent plasma polymerized coating of monomers such as propylene or siloxanes or silanes, either gaseous or liquid silanes may be employed, as well as organo-functional silanes or siloxanes. Also, fluorine containing monomers and organometallic monomers may be used. They may be in solid, liquid or gaseous forms, so long as they are vaporizable.
15(b). When the articles are to be over-coated with another plasma polymer layer such as siloxane, a good insulating layer can be achieved on top of the platinum conductive layer. The siloxane layer is also a good semi-permeable membrane layer which will allow the design of localized and controlled gas and/or chemical reactions to occur which combine the benefits of semi-permeable membranes with catalytic functionality.
15(c). Propylene monomers can also result in semi-permeable membrane overcoat layers and prior art has taught that plasma propylene membranes on top of platinum offer certain benefits for usage as electrodes, especially in hostile environments or for biomedical applications.
15(d). Various other copolymers and other plasma polymers known in the art can also be envisaged to be useful in selecting the rate of transfer of gases and/or chemicals to the platinum layer.
16. When the platinization of porous Vycor(trademark) is complete, i.e., when the preferred pre-exposure to organic vapors is followed by the preferred level of Argon treatment, the resulting platinum article has a resistance of only 30-500 ohms per cm. In certain circumstances, platinum articles having a lower resistance may be prepared. This resistance value indicates that the platinum is interconnected, yet the micropores in the Vycor(trademark) substrate remain intact throughout the metallized articles. By controlling the exposure time to the original monomer vapors one should be able to vary the amount of platinum deposition, as well as the pore sizes. The high surface area metallized membrane of the present invention can be used for catalytic conversions and separations. Reactions, such as oxidation, reduction, hydrogenation and other metal catalyzed reactions may be carried out with great ease by use of these metallized membranes.
17. By adding monomers such as propylene during or after first platinizing the articles, one can also envisage forming new platinum complexes for catalysis in chemical and biomedical applications.
By coating the platinum membrane with insulating coatings of silicon, fluoro or other such polymers or copolymers, one can envisage stable thin film conductors useful for electric current transmission.
The present invention further contemplates a method of controlling the amount of platinization and conductivity applied to respective substrates by exposing the substrates to precursor materials for varying amounts of time prior to plasma polymerization.
In other embodiments of the present invention, portions of the substrates may be masked prior to the plasma polymerization process, thereby causing only the exposed substrate areas to become polymerized. Examples of such masking techniques include masking the substrates in parallel ring patterns, and masking in longitudinal strips extending along length of the respective substrates. Various other masking patterns may be implemented as desired. Such substrates having specific polymerized areas may serve as ring electrodes or other various electro-catalytic elements. Other embodiments include strip arrays, strip electrodes, and various other applications.